The present invention is related to a multiple projector system. In this system multiple projectors are controlled and coordinated to provide a large display region such as a wall display. In such a display the goal is to provide a seamless image. However, in certain areas there is an overlap region where two projectors contribute light output to the same portion of the display surface. Unfortunately the combination from two projectors in this overlap region is additive and results in a brighter region with color differences. The overlap region is thus a visible band in the overall displayed image.
In order to minimize the visual band that occurs from the overlap region, a number of techniques have been proposed to “blend” the image to reduce the visual differences from one region to another. A simple blending method would output pixels in the overlapping edge regions of projector A and B at only 50% of their source brightness. Or, similarly, another simple method would have the pixels in the overlapping edge region of projector A be set to black (0% brightness) and pixels from overlapping edge region of projector B be left unchanged. Either method might conclude that the additive output will equate with 100% source brightness on the display surface.
However, this assumption incorrectly simplifies conditions that exist with actual projectors. With such an approach, boundaries between non-overlapped and overlapped regions (at the edges) require perfect alignment or results are visible as bright seams or gaps. Also, the extra light output by a single projector, even if emitting black, can affect the color blending enough to notice the edge boundaries. Device light output levels are seldom identical and so the 50%/50% approach cannot ensure success. Better blending is required to smoothly transition among non-overlapping and overlapping regions.
One better technique is to gradually reduce the output brightness for each projector pixel in the overlapping region in a reciprocal fashion. The brightness level of one projector gradual diminishes while the brightness of the other increases across the overlapping region. Each individual projector therefore contributes maximum brightness at its inside edge, equivalent to the non-overlapping regions nearest said edge, and contributes minimum brightness at its outside edge, furthest overlapped into the opposite projector at the boundary to the opposite projector's non-overlapped region. For example, pixels at the inside edges of the overlapping region are output at 100% brightness, while pixels at the outside edges are output at 0% brightness. This ensures that at any point between the edges, exactly 100% brightness will be achieved through a combination of the brightness of projector A plus the brightness of projector B. Since each logical pixel in the overlap region has some brightness value from either projector A or B, and no logical pixel contributes more than 100% brightness, there should be no seams or gaps.
Again, in actual practice, this better but still simple technique results in some visual bands or gaps in the image. Thus, in practice, the projectors are further adjusted using different blending formulas until the overall image looks fairly uniform. The terms “function,” formula,” and “algorithm” are used interchangeably herein to describe any method that blends or smoothes the overlap region formed between two projected images. As will be appreciated, there are an infinite number of blending formulas that can be used. But, there was no method available to determine the best formula for a particular situation, i.e., a particular set of projectors. Commonly owned U.S. patent application Ser. No. 12/501,162, filed Jul. 10, 2009, which is hereby incorporated by reference in its entirety, addressed the problem of determining the best formula for a particular situation. Commonly owned U.S. patent application Ser. No. 12/730,470 filed Mar. 24, 2010, which is hereby incorporated by reference in its entirety, addressed the problem creating blending ramps for complex projector image overlaps.
Digital display devices, such as LCD projectors, can generate a finite range of output colors. Commonly used is an RGB color model having 24-bits per pixel, composed of three color channels: red, green, and blue. This provides up to 256 color intensity levels for each 8-bit color channel, or 16,777,216 combined colors.
The multi-projector display system places high demands on this color range in the output projectors. Digital processes of all kinds stretch, compress, reshape, or otherwise manipulate source content and continually challenge color accuracy. To produce seamless large field displays in the best possible quality, a high bit depth is desirable.
The blending functions described in the above-referenced commonly owned patent applications, for example, operate on pixels in overlapping projector regions. However, even with so many color output choices, blending regions can experience trouble with producing smooth, visually continuous gradients of changing luminance. When a count of pixels in a blending region is greater than the range of available color values, the continuous gradient can appear as a series of discrete stair-steps. This banding, unwanted posterization (also solarization, false contouring effects), can even produce a Mach band optical illusion in which each band appears to have a light or dark region in a direction opposing the overall gradient. Some of these problems can be reduced by dithering or increasing bit depth.
Increasing the luminance in some display regions by electronically adding color values to more closely approximate brighter regions where projectors' displays overlap with one another is a method used by black level compensation. Better quality can usually be achieved when there are numerous low-luminance color values choices available. Projectors with very low black points are often more difficult to adjust as the range of color choices at such low luminance is small.
Blending, black level compensation, and many more requirements in a multi-projector display system quickly make use of the entire available color range.